Geology Reference
In-Depth Information
in these rocks is often very complex and heterogeneous depending upon the
fracture characteristics as discussed earlier. In fractured rocks, the groundwater
movement mainly takes place along discontinuities, i.e. joints, fractures and
shear zones. The interconnections between rock discontinuities and their
spacing, aperture size and orientation decide the porosity and permeability
of such rock masses. Open joints and fractures which are not filled with
weathered or broken rock material form potential passage for groundwater
movement but their permeability is greatly reduced when filled with clayey
material such as smectite or montmorillonite. These filling materials form
fracture skin which also influence the movement of solutes from the fractures
into the porous matrix.
Flow through Single Fracture
The Darcy's law for flow in a single fracture can be written as:
V
=
K
f
I
(1)
where
K
f
is the hydraulic conductivity of the fracture, defined by
2
a
K
f
=
(2)
12
where
a
is the fracture aperture,
is
the viscosity. The hydraulic conductivity (
K
), and permeability (
k
) are related
by the expression
is the specific weight of water and
k
K
=
(3)
Therefore, the permeability of the fracture,
k
f
, can be defined as:
a
2
k
f
=
(4)
12
By combining equations (1) and (2), the average velocity (
V
) in the
fracture expressed by a single parallel plate model is given by
2
a h
dl
=
(5)
V
12
Here it is assumed that the fracture walls are impermeable. In terms of
transmissivity of fracture,
T
f
, equation (4) can be written as:
2
a
Ka
T
f
=
(6)
f
12
Many researchers also defined
T
f
as the hydraulic conductivity of fracture.
The volumetric flow rate per unit plate (fracture) width (
Q
f
) will be
